Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device exhibiting improved emission efficiency.
Background Art
A Group III nitride semiconductor light-emitting device such as high brightness blue LED has been put into practical use. The light-emitting device has a double hetero structure in which a light-emitting layer having a MQW structure is sandwiched between an n-type cladding layer and a p-type cladding layer. Holes injected from the p-type cladding layer to the light-emitting layer hardly overflow from the n-type cladding layer because the holes are effectively confined in the light-emitting layer having a MQW structure due to a short diffusion length. Therefore, there is no need to form a potential barrier layer for the holes in the valence band as the function of the n-type cladding layer.
The inventors of the present invention came up with an idea that a strain relaxing layer having a multiplex period structure made of InGaN/GaN is formed just below the light-emitting layer, instead of the conventional n-type cladding layer for mainly confining holes therein, to improve the crystallinity of the light-emitting layer as shown in Patent Document 1.
Patent Documents 2 and 3 disclose that a light-emitting device having a layered structure of an AlGaN cladding layer, a GaN guide layer, a light-emitting layer (active layer) as an n-type layer side layer structure is known as a general structure.
In Patent Document 4, the electron injection efficiency to the active layer is improved by arranging a periodic structure of AlGaN/GaN/InGaN just below the active layer. Patent Document 4 discloses that two dimensional electron gases is accumulated at a hetero interface between the AlGaN layer and the GaN layer, and the electrons of high mobility are uniformly distributed in a plane surface, thereby improving the electron injection efficiency to the active layer. The strain is relaxed by decreasing the Al composition ratio as approaching the active layer.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2013-149938    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2011-151074    Patent Document 3: PCT International Application Publication No. WO2005/034301    Patent Document 4: Japanese Patent Application Laid-Open (kokai) No. 2012-222362
As described in Patent Document 1, when the strain relaxing layer made of InGaN/GaN is formed just below the light-emitting layer, the crystallinity of the light-emitting layer is improved, and the electron injection efficiency is increased because of low barrier for electrons. However, the electron density is highest at an interface between the light-emitting layer and the p-type cladding layer due to long diffusion length of the electrons, and a region having a high electron density is formed near the interface so that the electrons are not uniformly distributed in a thickness direction of the light-emitting layer. On the other hand, due to short diffusion length of the holes injected from the p-type cladding layer to the light-emitting layer, the hole density is highest near the interface between the light-emitting layer and the p-type cladding layer and is lowered toward the n-type cladding layer. Thus, the density of electron-hole pairs contributing to light emission is localized near the interface between the light-emitting layer and the p-type cladding layer. As a result, there are a lot of electrons not contributing to light emission, and the emission efficiency is conversely reduced.
In Patent Documents 2 and 3, an AlGaN cladding layer is formed on the n-type layer side, which serves as a barrier layer to block holes diffused in the light-emitting layer. However, the density of electrons injected into the light-emitting layer is not considered.
In Patent Document 4, an n-type cladding layer having a superlattice structure of AlGaN/GaN/InGaN is used to improve the electron injection efficiency by injecting two dimensional electron gas accumulated on the hetero interface into the active layer. However, as a result of improving the electron injection efficiency, unevenness occurred in a thickness direction of electron distribution in the light-emitting layer. A reduction in the emission efficiency due to the unevenness is not considered at all.
Therefore, an object of the present invention is to suppress the increase of drive voltage and improve the emission output and emission efficiency of a Group III nitride semiconductor light-emitting device.